The present invention relates to a brake release system and in particular, an all-hydraulic brake release system for use with heavy-duty vehicles having spring-activated, fluid pressure-released vehicle brakes.
Most engine-powered land vehicles have both a service braking system and a secondary braking system. The service braking system is designed for variable application during the vehicle's operation to control its speed and stop the vehicle when necessary. A separate and independent secondary or parking brake system is designed for stopping vehicle movement when the service braking system becomes disabled or when engine power is shut off.
In large heavy-duty materials handling vehicles, such as those used in construction and underground mining operations, spring-activated, fluid pressure-released secondary or parking braking systems are commonly used. One type of such a braking system is a driveline brake in which friction braking elements are spring-applied to the driveline and released by hydraulic brake fluid pressure tapped from the vehicle's hydraulic system. Thus, when hydraulic fluid pressure becomes unavailable to operate the vehicle's service brake system, either through failure of the service brake system or loss of engine power, the driveline brake is automatically spring-applied as a secondary or parking brake to stop the vehicle. Such a system, for example, has been used by Wagner Mining Equipment Co., of Portland, Ore., a division of PACCAR, Inc., for many years in its underground mining vehicles.
The application of the driveline brake is typically controlled by an electrical solenoid-operated control valve. However, electrical components such as these are subject to failure when used in corrosive atmospheres, such as commonly encountered in underground mining operations. In addition, the safety features provided by an electrically-controlled driveline brake can be easily deactivated or bypassed by the vehicle user, which is undesirable.
From the foregoing it will be apparent that there is a need for an all-hydraulic brake release system for use on heavy-duty vehicles. Therefore it is a primary object of the present invention to provide an all-hydraulic brake release system for use on vehicles having a spring-applied, fluid pressure-released brake.
It is a further object of the present invention to provide a brake release system which will only release the brake under normal vehicle hydraulic fluid operating pressures and which will automatically reapply the brake upon loss of normal vehicle hydraulic fluid operating pressure.
It is a further object to provide a brake release system which will reapply the brake upon loss of either brake fluid pressure or transmission clutch fluid pressure.
It is a further object of the present invention to provide a brake release system which is highly reliable, even in corrosive environments, such as found in underground mining operations.
It is a further object to provide a brake release system which has safety features which cannot be easily defeated by a vehicle operator.